1. Field
The present invention generally relates to DC blocking devices and improvements thereof. More particularly, the invention relates to DC block RF coaxial devices with surge protection and improvements thereof.
2. Description of the Related Art
DC block filters for use in electric circuits or between systems or devices are known and used in the art. Oftentimes in electrical systems, it is desirable to control input signal frequencies to a desired range of frequency values by blocking low frequency or DC signals from transmitting to a connected system or electrical component. Such signals can interfere with the designed operation of the connected system or damage the electrical components if not blocked along the transmission line. Devices, such as DC block filters, may be connected in-line along the transmission line to prevent the DC signals from encountering any connected equipment downstream from the filter.
Currently available DC block filters are commonly two-terminal devices and utilize a single capacitor connected in series between the two terminals. An input source is connected to one terminal and the hardware to be protected is connected to the other terminal. Depending upon the capacitor value of the DC block filter, certain voltage or current frequencies encounter a low impedance and are allowed to pass through the filter while other, lower frequency signals (e.g. DC signals) are blocked by the high impedance of the capacitor. Significant problems can arise if the capacitor of the DC block filter is damaged or otherwise fails and no longer operates to block the DC signals from reaching the connected hardware or equipment.
One particularly problematic cause of capacitor failure is the presence of a power surge on the transmission line utilizing the DC block filter. Power surges can originate from a variety of possible causes. One such cause is radio frequency (RF) interference that can couple to power or transmission lines from a multitude of sources. The power or transmission lines act as large antennas that may extend over several miles, thereby collecting a significant amount of RF noise from such sources as radio broadcast antennas. Another source of RF interference stems from equipment connected to the power or transmission lines that conducts along those lines to the equipment to be protected. In particular, older computer hardware may emit significant amounts of RF interference. A further cause of harmful electrical energy surges is conductive noise generated by equipment connected to the power or transmission lines which conducts along the lines to the equipment to be protected. Still another cause of disruptive electrical energy is lightning and typically arises when a lightning bolt strikes a component or transmission line that is coupled to the protected hardware or equipment. Lightning surges generally include DC electrical energy and AC electrical energy up to approximately 1 MHz in frequency and are complex electromagnetic energy sources having potentials estimated from 5 million to 20 million volts and currents reaching thousands of amperes.
Such electrical energy surges are often unpredictable and can significantly damage hardware or equipment either directly by entering the hardware or equipment via the transmission line or indirectly by damaging signal conditioning devices (e.g., DC block filters) connected in-line along the transmission line. Currently available DC block filters are particularly susceptible to such power surges since the incorporated capacitor is often not rated for high RF power and has a low breakdown voltage, for example of about 2 kV to 3 kV. The power surge, which can reach voltage levels of 20 kV or higher, will permanently damage the traditional DC block capacitor, often by shoot-through or punch-through of the capacitor dielectric or via carbon shorts. The surge will then continue to propagate down the transmission line towards any connected equipment. Incorporating a DC block capacitor with a much higher breakdown voltage to withstand the power surge is often not a viable solution since the use of such capacitors deteriorates the RF performance of the filter.
Even if the surge is mitigated by other surge suppression devices before reaching the connected equipment, the DC block filter will require replacement due to the permanent damage to the DC blocking capacitor. In certain cases, the failure of the DC block filter may not be readily apparent until the connected equipment begins to malfunction or fail due to the presence of unanticipated DC signal bias at its input. Contributing to the problem, communications equipment, computers, home stereo amplifiers, televisions and other electronic devices are increasingly manufactured using small electronic components that are increasingly vulnerable to damage from even small electrical signal variations outside the designed operating parameters. These signal variations can cost significant amounts in both damaged equipment or in maintenance costs to ensure filtering devices have not failed during their operation.
Therefore, a cost effective DC block device is needed to ensure hardware or equipment is adequately protected from undesirable DC signals even after a surge condition has propagated through the DC block device. Ideally, such a DC block device would have a compact size, a high return loss for passed RF signals, a low insertion loss for passed RF signals and a low voltage standing wave ratio (VSWR). In addition, the DC block device should be capable of continued operation to protect any connected equipment despite the occurrence of an electrical surge at the DC block device.